1. Field of the Invention
This invention relates to a thread-wound golf ball comprising a thread-wound core composed of a center ball and a layer of rubber thread thereon, and a cover formed over the core.
2. Prior Art
Thread-wound golf balls are conventionally made by winding highly stretched rubber thread onto a liquid center or a solid center to form a rubber thread layer about the center, and forming a cover of balata rubber or ionomer resin over the rubber thread layer.
Compared with two-piece solid golf balls, wound golf balls are preferred by professional golfers and skilled amateurs for their soft "feel" when hit with a golf club and their excellent spin performance (good spin receptivity). Yet, wound golf balls travel a steeper skying trajectory due to backspin, resulting in less carry than two-piece solid golf balls.
Because wound golf balls have a rubber thread layer in which the rubber thread has been stretched 8- to 10-fold and wound, their heat resistance is generally lower than that of two-piece solid golf balls. The possibility that the rubber threads may break under the effect of heat makes it difficult to form the cover by injection molding the cover stock over the wound core. Ways have thus been sought for preventing degradation of the rubber thread layer in the production of wound golf balls, and various solutions have hitherto been proposed.
For example, JP-A 47873/1980 and JP-A 115270/1982 describe the impregnation of a latex containing ionomer resin solids into the rubber thread layer surface, and the molding of the cover stock under applied heat and pressure.
The use of an injection molding process to form the cover in wound golf balls has also been proposed. For example, JP-A 112618/1986 and JP-A 112619/1986 describe the wrapping of a thermoplastic resin film over the rubber thread layer to form a protective layer on the surface thereof. When a cover stock, composed primarily of ionomer resin, is injected over the rubber thread layer, this film protects the rubber thread layer from the heat of the cover stock. However, the cover stock injected here is composed primarily of ionomer resin, which has a relatively low melting point close to 90.degree. C. but a much higher plasticizing temperature of 200.degree. C. Injection molding cannot be carried out without raising the resin temperature to 200.degree. C. Thus, although the above-described treatment of the rubber thread layer surface holds down breakage of the rubber threads, the hot cover stock compromises the properties of the rubber thread through the protective layer or latex, leading to declines in the hardness and initial velocity of the golf ball.
A number of attempts have been made to develop wound golf balls having greater carry. One attempt is to increase the moment of inertia of the golf ball.
The moment of inertia of a golf ball exerts a large influence on such properties during the flight of a golf ball as the trajectory, carry and control of the ball. Increasing the moment of inertia generally serves to lower the attenuation of spin during flight of the ball so that the spin rate is maintained even as the ball passes the peak of its trajectory and descends, making for an elongated trajectory. Moreover, when the ball is putted on a green, a higher moment of inertia increases the straightness of the shot and improves the roll.
Hence, a number of golf balls having large moments of inertia have been proposed (e.g., JP-B 73427/1993, JP-A 129072/1984, and JP-A 210272/1985). More particularly, the moment of inertia is increased by using a cover stock of ionomer resin having blended therein a high specific gravity filler such as white barium sulfate or titanium oxide as disclosed in JP-A 61-290969/1986.
However, because the filled cover stock is less flowing, the cover stock does not readily penetrate the rubber thread layer in the case of wound golf balls, which sometimes results in a lower durability. In addition, other problems include a decrease in resilience and reduced carry, as well as burring and napping of the cover.
Attempts have also been made in which heavy fillers having a specific gravity of 8 or more such as tungsten are blended into the cover formulation. There are limits to the adjustments that can be made by blending in weight-modifying ingredients. In addition, the resulting cover is not satisfactorily white.
Cover resins have also been the subject of various investigations. Thermosetting polyurethane elastomers are often used as substitutes for balata rubber or ionomer resin because of their relatively low cost and their good feel and scuff resistance (e.g., U.S. Pat. Nos. 4,123,061, 3,989,568, and 5,334,673).
Such thermoset polyurethane elastomers are superior in terms of scuff resistance, which is a shortcoming of soft blends of ionomer resins. However, after the starting materials have been poured, curing reactions and other complex operations must be carried out, making the adaptation of this technology to mass production quite difficult. Moreover, when only aliphatic isocyanate is used in the thermosetting polyurethane elastomer, the curing reaction rate is too slow. The use of some aromatic isocyanate is desirable for speeding up the reaction rate. The use of aromatic isocyanate, however, causes the cover to yellow with time. Even if a white enamel coating is applied to the outside of the ball to hide this, the appearance and color of the ball deteriorate as the urethane cover yellows.
Covers made of thermoplastic polyurethane elastomer have also been investigated (e.g., U.S. Pat. Nos. 3,395,109, 4,248,432 and 4,442,282). Although thermoplastic polyurethane elastomers improve the scuff resistance when the ball is hit with an iron club, as well as the moldability and other properties, there has yet to be obtained a sufficient improvement in flight distance due to an increased moment of inertia. Hence, the development of a golf ball with a thermoplastic polyurethane elastomer cover having even higher performance and quality has been awaited.
On the basis of studies aimed at improving the performance of wound golf balls by enhancing the moment of inertia, the present inventors proposed in U.S. Ser. No. 08/841,559 now U.S. Pat. No. 5,800,286 and Ser. No. 08/841,677 now U.S. Pat. No. 5,792,008 which are assigned to the same assignee as the present invention, golf balls with covers in which the primary component is a non-yellowing thermoplastic polyurethane elastomer. Owing to the increased moment of inertia, these wound golf balls offer a longer carry and excellent control, as well as excellent scuff resistance on iron shots, yellowing resistance, and moldability. Even so, there remains a desire for wound golf balls having even higher performance and quality.